The present invention relates generally to cobalt-chromium (Co-Cr) based orthopedic implant devices and, more particularly, to a surface hardening process applicable to such devices, wherein surface hardness and wear resistance properties of the implant are enhanced with minimal loss in fatigue strength.
Cobalt-chromium based alloys have been used for orthopedic applications because of their strength, corrosion resistance, and biocompatability. However, under conditions of sliding wear or articulation of the cobalt-chromium alloy against other surfaces, particularly polymers, metals, ceramics, bone, and bone cement, the cobalt-chromium alloy will produce wear debris from articulating surfaces. Therefore, the surface of the alloys must be hardened in order to minimize wear.
In the past, surface hardening of cobalt-chromium based orthopedic implants has been achieved by depositing a titanium nitride coating on the surface of the implant, or by ion implantation of the cobalt-chromium substrate. Known surface hardening methods include gas nitriding, chemical salt bath nitriding, plasma or ion nitriding, and ion implantation. Of these methods, gas nitriding exhibits advantages over the other methods in terms of cost and ease of manufacture. For example, gas nitriding permits efficient batch processing of many parts concurrently in a furnace chamber; whereas, the ion implantation method requires line-of-sight bombardment of the workpiece, thereby limiting the dose uniformity and the number of parts that may be processed concurrently.
Gas nitriding of titanium based implants is well known. For example, surface hardening of titanium and its alloys has historically been performed by gas nitriding at elevated temperatures in the range of 700.degree. C. to 1200.degree. C. (1292.degree. F. to 2192.degree. F.). However, due to undesirable changes of certain physical and mechanical properties of the alloy based upon such high temperatures, a more recent process has been developed for surface hardening titanium alloys at a temperature of about 1100.degree. F. for approximately eight hours. The hardened surface of the titanium implant occurs primarily due to solid solution hardening of the titanium with nitrogen by dissolution. This process is disclosed in detail in U.S. Pat. No. 5,912,323 issued on Mar. 9, 1993, and entitled "METHOD OF SURFACE HARDENING ORTHOPEDIC IMPLANT DEVICES", which disclosure is hereby incorporated by reference.
It is also known that cobalt-chromium based alloys, specifically ASTM F-75 and ASTM F-799 alloys, can be strengthened by adding nitrogen into the alloy in the molten state or diffusing nitrogen into the alloy in the solid state. Specifically, forming gas (15% hydrogen, 85% nitrogen) is utilized in combination with either ammonia or argon. However, such processes actually change the chemistry of the alloy by significantly increasing the weight percent of nitrogen present throughout the alloy. It has been found that when nitrogen is added in this manner, the fracture toughness of the alloy is reduced significantly because of the gross change in the chemistry of the alloy. It has also been found that hydrogen embrittlement and decarburization results from the hydrogen which is present in large amounts in dissociated (cracked) ammonia. If decarburization occurs, carbon is lost from the surface of the alloy, thereby decreasing the hardness of the alloy. Consequently, it is desirable to enhance the surface hardness of the cobalt-chromium material without substantial losses in fatigue strength or wear resistance properties.